In the design of multi-color printing presses wherein the same sheet is fed a number of times into the press to achieve the desired coloring, it is well known that extremely precise front and lateral registry of the rectangular stock must be achieved and maintained with respect to the printing system prior to engagement with the printing elements. Additionally, when printing transparencies, stock pre-printed on one face must be turned over for one or more additional passes through the printing press. Here accurate front-to-rear registration is essential, again requiring suitable end and edge alignment systems.
Front end alignment is achieved in conventional printing presses by pressing the front edges of each sheet against a stop in the press. These commercial presses do not however generally provide a lateral edge alignment operation. Reliable side edge alignment, especially where lateral alignment is to be achieved of the upper most sheet in a stack of sheets, presents a difficult problem where it is desired to minimize costs in establishing such lateral edge alignment, and where both the cost of the press as well as that of the sheet feeding equipment and their reliability are paramount requirements in marketing the presses.
One procedure has been to mount the stack against a side alignment stop, typically configured in the form of a rail extending along the feed direction into the press. Sometimes the upper portion of a stack is spring urged against an edge alignment stop; however, consistently good side edge alignment is difficult to achieve in this manner, since erratic standoff distances are frequently encountered, commonly causing alignment errors of as much as 0.015" (0.28 millimeters). Other prior art alignment systems are either unreliable or too costly for applications where low cost is a paramount factor.
One prior lateral edge sheet alignment system (U.S. Pat. No. 2,819,078 to DuRand), uses a motor driven friction rotor disposed to engage the upper surface of the sheet and drive it sideways against a side alignment stop. One principal disadvantage of such a roller is that if it presses sufficiently hard to successfully move the sheet against the side alignment stop, a certain amount of scuffing occurs on the upper surface of the top sheet. This will frequently cause smearing of pre-printed inked areas as a result, and so this roller feed operation is not suitable for use with multi-color printing presses.
Another problem that such alignment systems must cope with arises from the fact that the corner angles of sheet stock as received from a manufacturer are seldom an exact 90.degree., and may in fact may vary from this by as much as 3.degree.. In such cases if the edge alignment system aligns a given reference edge exactly perpendicular to, for example, the axis of rotation of the print and backing rollers in a rotary press, the leading edge of the sheet will be offset when intercepted by the stop in the press, and tilting of the sheet can result, causing color dot misalignment. This raises serious problems during two-sided printing operations, since reprinting a given sheet of stock on the reverse side using the same guide orientation will result in serious runout errors and general loss of color registry along the sheet.
There thus remains a need for an alignment system which will reliably and gently feed a top sheet of a stack against a side aligning stop without scuffing the surface of the paper. Additionally, there is a need for providing a side alignment stop which can accommodate the above mentioned errors in the corner angles of commercially available stock to allow accurate side alignment of the stock during reverse side printing operations.
One solution to some of the foregoing problems is shown in U.S. Pat. No. 4,591,143 (Jeschke). This patent shows a single sheet alignment system having end stops for securing leading edge alignment of an inserted sheet. A pair of motor driven vacuum platforms mounted in apertures in a sheet support platform drives a pair of vacuum chucks mounted for coaxial reciprocating movement beneath the lower surface of an emplaced sheet. The drive motors are stepping motors, and they are advanced initially in opposite directions to tension the leading edge of the sheet, whereupon they jointly proceed to move the sheet by increments until one edge is detected by an edge sensor. This terminates the alignment operation or initiates a fixed distance adjustment operation. The vacuum is released prior to pickup of the leading sheet edge by the feed mechanism of the press. The principal disadvantage of this system is that it is too expensive for many applications, in particular for low cost printing installations.
The instant invention is designed to solve these and related problems.